Method for producing glass beads for electrostatographic developers

ABSTRACT

Spherical glass beads are produced by isolating individual glass particles on a non-wetting ceramic block and heating the glass to a temperature at which the glass particles are drawn-up into spherical beads. The spherical beads, either coated or uncoated, may be employed as the carrier in an electrostatographic developer. The carriers are characterized by more uniform sizing and greater approximation to a spherical shape.

United States Patent Hagenbach [4 1 Apr. '25, 1972 [54] METHOD FORPRODUCING GLASS 2,739,348 3/1956 Rayburn ..65/21 x BEADS FORELECTRQSTATOGRAPHIC 3,341,314 9/1967 Vukasovich et al. .....65/2l XDEVELOPERS 2,4619

Appl. No.: 868,903

U.S. Cl. ..65/21, 96/1 R, 252/62.l Int. Cl. ..C03b 37/00 Field of Search..65/21, 142; 252/62. 1 96/ 1 References Cited UNITED STATES PATENTSLeighton ..65/21 2/1949 Taylor et al ..65/142 UX Primary Examiner-S.Leon Bashore Assistant Examiner-Robert L. Lindsay, Jr.

Attorney-James J. Ralabate, Albert A. Mahassel, Peter H. Kondo, Mam &Jangarathis, James J. Burke, 11 and R. G. Rhodes [57] ABSTRACT Sphericalglass beads are produced by isolating individual glass particles on anon-wetting ceramic block and heating the glass to a temperature atwhich the glass particles are drawnup into spherical beads. Thespherical beads, either coated or uncoated, may be employed as thecarrier in an electrostatographic developer. The carriers arecharacterized by more uniform sizing and greater approximation to aspherical shape.

4 Claims, No Drawings BACKGROUND OF THE INVENTION This invention relatesto electrostatography and more particularly to an improved process forproducing spherical glass beads for use as a carrier in aelectrostatographic developer.

Xerography is exemplary of an electrostatographic process and the basicxerographic process, as taught by C. F. Carlson in U.S..Pat. No.2,297,69l, involves placing a uniform electrostatic charge on aphotoconductive insulating layer, exposing the layer to'a light andshadow image to dissipate the charge on the areas of the layer exposedto the lightand developing the resulting latent electrostatic image bydepositing on the image a finely-divided electroscopic material referredto in the art as toner. The toner is normally attracted to those areasof the layer which retain a charge, forming a toner image correspondingto the latent electrostatic image, which may then betransferred to asupport surface, such as paper. The transferred image is generallypermanently affixed to the support surface by heat, although othersuitable fixing means, such as solvent or overcoating treatment, may besubstituted for the foregoing heat fixing steps.

Many methods are known for applying the electroscopic particles to thelatent electrostatic image to be developed. One development method asdisclosed by E. N. Wise in U.S. Pat. No. 2,618,552 is known as cascadedevelopment. In this method, developer material comprising relativelylarge carrier particles having finely-divided toner particleselectrostatically clinging to the surface of the carrier particles isconveyed to and rolled or cascaded across the latent electrostaticimage-bearing surface. The composition of the toner particles is sochosen as to have a triboelectric polarity opposite that of the carrierparticles. In order to develop a negatively charged latent electrostaticimage, an electroscopic powder and carrier combination should beselected in which the powder is triboelectrically positive in relationto the carrier. Conversely, to develop a positively charged latentelectrostatic image, the electroscopic powder and carrier should beselected in which the powder is triboelectrically negative in relationto the carrier. This triboelectric relationship between the powder andcarrier depends on their relative positions in a triboelectric series inwhich the materials are arranged in such a way that each material ischarged with a positive electrical charge when contacted with anymaterial below it in the series and with a negative electrical chargewhen contacted with any material above it in the series. As the mixturecascades or rolls across the image-bearing surface, the toner particlesare electrostatically deposited and secured to the charged portions ofthe latent image and are not deposited on the uncharged or backgroundportions of the image. Most of the toner particles accidentallydeposited in the background are removed by the rolling carrier, dueapparently, to the greater electrostatic attraction between the tonerand carrier than between the toner and the discharged background. Thecarrier particles and unused toner particles are then recycled. Thistechnique is extremely good for the development of line copy images. Thecascade development process is the most widely used commercialxerographic development technique, a general purpose office copyingmachine incorporating this technique is described in U.S. Pat. No.3,099,943.

In the cascade development technique, the carrier particles are rolledacross the image-bearing surface so that their gravitation or momentumforce is greater than the force of attraction of the toner in the areasof the image-bearing surface retaining the toner to prevent the carrierparticles from adhering to the retained toner particles. Thus, thecarrier particles should be capable of flowing easily over theimage-bearing surface, without the necessity of providing special meansfor effecting removal of the carrier material from the image-bearingsurface.

Glass beads are commonly employed in electrostatographic developercarriers and in order to facilitate rolling of the carrier across animage-bearing surface, such beads should be substantially spherical.

SUMMARY OF THE INVENTION An object of this invention is to provide animproved process for producing spherical glass beads for use in anelectrostatographic developer.

Another object of this invention is to provide an improved process forproviding an electrostatographic developer carrier.

A further object of this invention is to provide for the production ofspherical glass beads of a more uniform size.

Yet another object of this invention is to provide for improveddevelopment of a latent electrostatographic image.

These and other objects of the invention should be more readily apparentfrom reading the following detailed description thereof. 7

The objects of this invention are broadly accomplished by placing finelydivided glass on a non-wetting ceramic support in a manner such thatindividual glass particles are isolated from each other, followed byheating of the glass to a temperature at which the particles are drawnup into substantially spherical beads as a result of their surfacetension. The spherical glass beads are .used in an electrostatographicdeveloper carrier.

DETAILED DESCRIPTION OF THE INVENTION In accordance with the invention,the glass employed as a starting material may be any one of a widevariety of glasses, including soda-lime-silica base glasses, high leadglasses, barium glasses and the like, and may be employed as a startingmaterial in either solid or molten form. If employed as a solid, theglass is screened to the desired size and if employed in molten form,the desired sizing is obtained by uniformaly shearing the desired sizefrom a controlled molten glass stream which is extruded from acontinuous glass melting furnace. Al-

. ternatively, the desired sizing of molten glass may be achieved byselecting an appropriate orifice size for a glass melting furnace.

The particle size of the solid or molten glass employed as the startingmaterial controls the particle size of the spherical glass beadproduced. In general, the carrier material of xerographic developershould have a particle size from about 30 to about 1,000 microns, andtherefore the particle size of the glass starting material is controlledto provide a final product having a particle size suitable for axerographic developer carrier.

The glass starting material, either in solid or molten form, having theappropriate particle size, is placed on a non-wetting ceramic block,i.e., a ceramic block to which the glass does not stick, in a mannersuch that the individual particles do not touch each other, therebypreventing fusion of two or more particles which reduces the productionof individual spherical particles. The non-wetting ceramic materialswhich may be employed in producing the block, include, as representativeexamples; graphite, chrome oxide, iron oxide, zirconium oxide, aluminumoxide, ball clays, etc.

In accordance with a particularly preferred embodiment of the invention,the surface of the ceramic block on which the glass particles are to beplaced, includes a plurality of small impressions of a size suitable forholding an individual glass particle, whereby each individual particleis effectively separated from the adjacent particles. The impressionspreferably have a conical shape to facilitate production of sphericalbeads and may be arranged on the surface of the block in any manner,provided the impressions are spaced from each other by a distancesufficient to isolate each individual glass particle. The forming ofsuitable impressions on the block for isolating individual glassparticles is deemed to be within the scope of those skilled in the artfrom the teachings herein, and therefore no further description isdeemed necessary for a full understanding of the invention.

The glass particles, which are isolated from each other on thenonswetting ceramic block, are heated to a temperature at which theviscosity of the glass is sufficiently reduced to permit the surfacetension to draw the glass particle into a spherical bead. Thetemperature to which the glass particles are heated varies with theparticular glass material used and the choice of a suitable temperaturefor causing the glass to drawup into spherical beads is deemed to bewithin the scope of those skilled in the art from the teachings herein.In general, the glass particles are heated to temperatures from about1,200 F. to about 2,210 E, preferrably from about l,700 F. to about1,850 E, and at such temperatures, the glass particles are rapidlyfromedinto spherical beds; generally in the order of from about 2 to about 5minutes.

The heating of the glass particles may be efiected in any one of a widevariety of furnaces heated by any one of a wide variety of heat sourcesand the choice of a particular heating apparatus is well within thescope of those skilled in the art. In a preferred procedure, thespherodizing procedure is effected in a continuous manner by the use ofa controlled tunnel furnace, of a type known in the art, includingpreheat, spherodizing, annealing and cooling zones. The non-wettingceramic block, containing the sized glass particles, is sequentiallyconveyed through the furnace zones, providing improved control over theoverall process.

After the glass beads are formed on the ceramic block, the beads areallowed to cool while on the block, to a temperature at which the beadsare solidified, i.e., the beads are not at a temperature at which thebeads will adhere to each other or other particles. In general, thebeads are initially cooled to a temperature from about 1,800 F. to about610 F. or lower, and are then placed in suitable containers forsubsequent classification as to size.

The spherical glass beads produced, as hereinabove described, may thenbe employed as the core material of an electrostatographic developercarrier as generally known in the art. Thus, the glass beads may beencased in a suitable covering which imparts the triboelectricproperties required for an electrostatographic developer carrier, i.e.,the carrier properly charges the toner when mixed therewith. The coatingmaterial for the glass beads may be any one of the wide variety ofcoating materials employed as coatings in electrostatographic developercarriers, generally resinous material including, but not limited to:polyolefins, such as polyethylene, polypropylene, etc.; vinyl andvinylidene resins, such as styrene, vinyl chloride, vinyl acetate,acrylonitrile, methyl methacrylate and like resins; phenolic resins,such as phenolformaldehyde, etc.; aminoresins, such asmelamine-formaldehyde; and the like, and the coating may be applied byany one of a wide variety of procedures, such as spraying, dipping, afluidized bed coating, tumbbling, brushing and the like. In accordancewith one procedure, the glass beads are coated with an intermediatebonding layer, and the coated core added to the dry resinous material inwhich the glass bead is to be encased, whereby the resinous casingmaterial adheres to the coated glass bead and constitutes a coatingwhich is fused or otherwise affixed to the core material. The productionof electrostatographic carriers is further described in U.S. Pat. No.2,618,551 to Walkup which is hereby incorporated by reference.

The spherical glass beads produced in accordance with the invention mayalso be employed as an uncoated electrostatographic developer carrier.Thus, for example, glass compositions, as disclosed in copending U.S.application Ser. No. 631,192 filed on Apr. 17, 1967, now U.S. Pat. No.3,591,503 by Hagenback et al., are suitable for the production ofuncoated carriers and such glasses may be formed into spherical glasscarriers in accordance with the teachings of the invention.

The toner particles of the developer may be any one of the wide varietyof toner materials generally employed in developer compositions and asrepresentative examples of typical toner materials, there may bementioned: gum copal,

gum sandarac, rosin, cumaroneindene resin, asphaltium, gilsonite,phenolformaldehyde resins, rosin-modified phenol-formaldehyde resins,methacrylic resins, polystyrene resins, polypropylene resins, epoxyresins, polyethylene resins and mixtures thereof. Among the patentsdescribing electroscopic toner compositions are U.S. Pat. No. 2,659,670to Copely; U.S. Pat. No. 2,753,308 to Landrigan; U.S. Pat. No. 3,079,342to lnsalaco; U.S. Pat. Reissue No. 25,136 to Carlson and U.S. Pat. No.2,788,288 to Rheinfrank et al.

The toner contains a pigment or dye in a quantity sufficient to impartcolor to the toner, generally in an amount from about 1 percent to about10 percent, by weight, of the toner. Any one of a wide variety ofpigments or dyes which do not adversely affect the properties of thetoner may be employed to impart color to the toner particles; e.g.,carbon black, a commercial red, blue or yellow dye, and since such dyesand/or pigments are well known in the xerographic art, no detailedenumeration of such dyes or pigments is deemed necessary for a fullunderstanding of the invention. The toner is prepared by thoroughlymixing the various components to fonn a uniform dispersion of the dye orpigment in the toner material and thereafter the toner material isfinely divided; e.g., to a particle size of less than about 30 microns,preferrably from about 2 to about 10 microns in average particle size.The preparation of toner compositions in this manner is well known inthe art and therefore no detailed description thereof is deemednecessary for a full understanding of the invention.

The toner particles are mixed with the carrier, acquiring a chargehaving an opposite polarity to the carrier, whereby the toner particlesadhere to and surround the carrier. The degree of contrast or otherphotographic qualities of the finished image, may be varied in part, bychanging the relative proportions of carrier material and toner, withthe choice of optimum proportions being well within the scope of thoseskilled in the art. In general, however, the ratio of carrier to tonervaries from about 250:1 to about 25:1, depending on the toner materialcarrier.

The electrostatic developer produced in accordance with the inventionmay then be employed for developing a latent electrostatic image inaccordance with procedures wellknown in the art. Thus, for example, thedevelopers of the invention are particularly suitable as developers inthe cascade" development technique, hereinabove described, employed in axerographic process.

The invention is further described with reference to the followingexamples, which are illustrative of preferred embodiments thereof, butthe scope of the invention is not to be limited thereby.

EXAMPLE 1 A crushed soda-lime glass ranging in size from 20-35 mesh isplaced on a non-wetting ceramic block formed of chrome oxide in a mannersuch that the individual particles to not touch each other. The block isplaced in a controlled mufile furnace operated at a temperature of l,850F. and retained in the furnace for a period of 5 minutes to permit theglass particles to soften to the point at which the surface tensioncauses the glass particles to form a spherical shape.

The spherical glass beads are removed from the furnace, cooled andencapsulated in a resinous coating, as described in Example I of U.S.Pat. No. 2,613,551 to Walkup.

The coated carrier is then mixed with a toner comprising astyrene-n-butyl methacrylate copolymer, polyvinylbutyral and carbonblack, produced by the method disclosed in Example I of U.S. Pat. No.3,079,342 to lnsalaco, to provide a composition containing about 1percent toner, by weight. The cascading of the developer over alatent-electrostatic image formed on a xerographic plate provides gooddevelopment with minimal background deposition.

Crushed Flint glass ranging in size from 20 25 mesh is placed on anon-wetting ceramic block formed of graphite in a manner such that theindividual particles do not touch each other. The block is placed in acontrolled muffle furnace operated at a temperature of 1,350 F. andretained in the furnace for a period of four minutes to permit the glassparticles to soften to the point at which the surface tension causes theglass particles to form a spherical shape. The spherical glass beads areremoved from the furnace and cooled.

The glass beads are then mixed with a toner comprising a styrene-n-butylmethacrylate copolymer, polyvinylbutyral and carbon black, produced bythe method disclosed in Example I of US. Pat. No. 3,079,342 to lnsalacoto provide a composition containing about 0.5 percent toner, by weight.The cascading of the developer over a latent-electrostatic image formedon a xerographic plate provides good development with minimal backgrounddeposition.

The glass beads produced in accordance with the invention are clean andof a more uniform size than those presently produced in commercialprocesses. As a result of the more uniform sizing and nearerapproximation to a spherical shape, the use of the glass beads eithercoated or uncoatedQas an electrostatographic developer carrier enablesthe developer to be more easily rolled across an image-bearing surface,thereby facilitating effective transfer of toner to the image-bearingsurface.

Numerous modifications and variations of the present invention arepossible in light of the above teachings and therefore the invention maybe practised otherwise than as particularly described.

What is claimed is: 1. A process for producing spherical glass beads foruse as an electrostatographic developer carrier comprising:

heating stationary glass particles isolated from each other in aplurality of small impressions in a non-wetting ceramic support, asingle glass particle being placed and heated in a single impression,said glass particles being of a particle size. to provide spheres havinga particle size from about 30 microns to about 1,000 microns, saidheating being effected to a temperature at which the glass particles aredrawn-up into spherical glass beads; cooling the beads; and recoveringthe beads. 2. The process as defined in claim 1 wherein the glassparticles are in solid form.

3. The process as defined in claim 1 wherein the glass parti-

2. The process as defined in claim 1 wherein the glass particles are insolid form.
 3. The process as defined in claim 1 wherein the glassparticles are in molten form.
 4. A process as defined in claim 1 whereinthe glass particles are heated to a temperature from about 1,200* F. toabout 2,210* F.